1. Field of the Invention
The present invention relates to an epitaxial wafer, a method for producing the epitaxial wafer, a photodiode, and an optical sensor device. Specifically, the present invention relates to an epitaxial wafer, a method for producing the epitaxial wafer, a photodiode, and an optical sensor device that include a multiple-quantum well (MQW) formed of III-V compound semiconductors and having a bandgap corresponding to the near- to far-infrared region.
2. Description of the Related Art
InP-based semiconductors, which are formed of III-V compounds, have a bandgap energy corresponding to regions ranging from the near-infrared region to the far-infrared region and hence a large number of studies are performed for developing photodiodes for communications, image capturing at night, and the like. For example, R. Sidhu, et al. “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells”, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717 and R. Sidhu, et al. “A 2.3 μm Cutoff Wavelength Photodiode on InP Using Lattice-Matched GaInAs-GaAsSb Type-II Quantum Wells”, 2005 International Conference on Indium Phosphide and Related Materials, pp. 148-151 describe a photodiode in which an InGaAs/GaAsSb type-II MQW is formed on an InP substrate, the photodiode having characteristic sensitivity in the near-infrared region. Japanese Unexamined Patent Application Publication No. 2009-206499 proposes a planar photodiode in which an InGaAs/GaAsSb type-II MQW is formed on an InP substrate and zinc (Zn) serving as a p-type impurity is selectively diffused through a selective diffusion mask pattern, so that pixels are formed so as to be isolated from each other with non-diffusion regions therebetween. On this MQW, an InP window layer and an InGaAs diffusive-concentration-distribution-adjusting layer are disposed. The configuration of such a planar photodiode eliminates the necessity of performing etching for forming a mesa structure and, as a result, allows a decrease in the dark current.